1. Field of the Invention
This invention relates to the field of actuator motor assemblies for disk drives.
2. Art Background
Hard disk drives use aluminum disks to store blocks of data. The disks have magnetic surfaces and store data in blocks that lie along concentric circular tracks. The data is stored and recovered by read/write heads. These heads do not touch the surface of the disk, but ride on what is known as an "air bearing". Typically the heads are suspended 17-20 microinches from the surface of the disk.
The read/write heads are located on the end of a driver arm whose function is much like the tone-arm of a record player. However, since the heads of a disk drive are not touching the surface of the disk, an actuator motor provides an independent source of motion of the heads across the surface of the disk.
In order to optimize storage capacity, information is stored on both sides of a hard disk. Two heads are mounted on the drive arm, one for each side of the disk. The disk rotates at approximately 3000 rpm and contains a high density of storage tracks. For example, the present invention contemplates a hard disk having 600 tracks per inch.
Hard disk drives are typically used as storage devices for a host computer. When a read/write command is received from the computer, the drive arm is moved across the disk surface, seeking the appropriate track to access. The drive arm is propelled by an actuator motor.
The actuator motor consist of a fixed magnet and a coil attached to the arm. Additional weight is added to the arm to counterbalance the arm, much like the counterweight on a record player tone arm. The driver arm is counterbalanced in order to equalize the moment of each side of the arm. The moment of the portion of the arm extending on one side of the pivot point is dependent on the mass of the portion and the radius through which it acts. By counterbalancing the driver arm, the moments of each side of the pivot point are equal, and the arm can be operated in any plane without the need for complex servo circuitry. When the moment on each side of the pivot point is equal, the arm will not rotate unless additional force is applied to one side. Thus, changes in the plane of the arm, and shocks to the disk drive, will not cause the arm to rotate, reducing the chance of damage during shipping and other movement.
When the driver arm is swept across the surface of the hard disk, a certain amount of inertia is built up in the arm. In order to stop the arm directly over a track, the arm must begin slowing prior to reaching that track, to overcome the inertia. The time spent slowing the arm adds to the access time of the disk drive. The inertia is a function of mass multiplied by the square of the radius through which it acts. A disadvantage of prior art driver arm assemblies is the added mass of the counter balance and the larger radius through which it acts. In prior art disk drives, the counterweight must be placed outside of the coil/magnet structure, increasing the radius and the size of the arm. The increased radius means greater inertia. This added inertia not only reduces the access time, it also requires a more powerful actuator motor using more current to supply the starting and stopping force for the driver arm. A larger motor means higher cost and larger space requirements, while higher current requirements result in additional heat and higher operating costs. Further, the counterbalance limits the reduction in size of the entire disk drive assembly.
The present invention seeks to overcome the disadvantages of prior art driver arms by providing a driver arm which incorporates, as part of its structure, the motor assembly for sweeping it over the disk. Accordingly, the present invention does not require separate counterweights for the driver arm, and a lower power motor may be used. Additionally, the arm can be made a shorter length than prior art driver arms, allowing for more compact disk drives.